This invention relates to a dielectric material suitable for use as a dielectric resonator. More particularly, the invention relates to a low loss dielectric material suitable for use as a high frequency dielectric resonator.
Materials of the BaOxe2x80x94MgOxe2x80x94Nb2O5 system (referred to herein as the BMN system) are known as high frequency dielectric materials. Laid-Open Japanese Patent Application No. 60-124305 and Japanese Patent Publication No. 2-60628 describe materials of the BMN system suitable for high frequency use.
However, the BMN system materials exemplified in the above Japanese patent documents include Ta which is expensive. In order to reduce manufacturing costs, it would be desirable to provide a BMN system material containing no Ta but still having desirable high frequency characteristics.
The present invention has as an object thereof to provide a dielectric composition which is based on a BMN system material but which includes no Ta, and in which: (1) the dielectric constant xcex5 is about 30, (2) the Q-value, i.e., the no-load quality coefficient, is large, and (3) the absolute value of xcfx84f, the temperature coefficient of the resonant frequency, is comparatively small. As will be understood by those skilled in the art, the parameters Q (sometimes given as Q0) and xcfx84f are important quantities in analyzing the characteristics of a dielectric material, with the latter being determined by measuring the change in resonant frequency with temperature.
In accordance with a first aspect of the present invention, there is provided a dielectric material having a composite perovskite crystal structure including K, Ba, Mg and Nb as metallic elements in a main crystal phase, and having a compositional formula represented by:
(1xe2x88x92x)Baxcex1(Mgxcex2Nb1xe2x88x92xcex2)O3xe2x88x92xKpNbO3. 
wherein x, xcex1, xcex2 and p have values satisfying the conditions
0 less than xxe2x89xa60.1, 0.9xe2x89xa6xcex1xe2x89xa61.3, 0.3xe2x89xa6xcex2xe2x89xa60.35 and 1xe2x89xa6pxe2x89xa62. 
In accordance with a second aspect of the present invention, there is provided a dielectric material having a composite perovskite crystal structure including K, Mg, Sb, Ba and Nb as metallic elements in the main crystal phase, and having a compositional formula represented by:
(1xe2x88x92x)Baxcex1(Mgxcex2Nbxcex3Sbxcex4)O3xe2x88x92xKpNbO3, 
wherein x, xcex1, xcex2, xcex3, xcex4 and p have values satisfying the conditions
0 less than xxe2x89xa60.1, 0.9xe2x89xa6xcex1xe2x89xa61.3, 0.3xe2x89xa6xcex2xe2x89xa60.35, 0 less than xcex4xe2x89xa60.125, xcex2+xcex3+xcex4=1 and 1xe2x89xa6pxe2x89xa62. 
In accordance with a third aspect of the present invention, there is provided a dielectric material having of a composite perovskite crystal structure including Sn, K, Mg, Sb, Ba and Nb as metallic elements in a main crystal phase, and having a compositional formula represented by:
(1xe2x88x92x){(1xe2x88x92y)Baxcex1(Mgxcex2Nbxcex3Sbxcex4)O3-yBaSnO3}-xKpNbO3 
wherein x, y, xcex1, xcex2, xcex3, xcex4 and p have values satisfying the conditions
0 less than xxe2x89xa60.1, 0 less than yxe2x89xa60.5, 0.9xe2x89xa6xcex1xe2x89xa61.3, 0.3xe2x89xa6xcex2xe2x89xa60.35, 0 less than xcex4xe2x89xa60.125, xcex4+xcex3+xcex4=1 and 1xe2x89xa6pxe2x89xa62. 
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.
In the compositional formulas set forth above, the oxygen ratios will naturally depend upon the variables xcex1, xcex2, xcex3, xcex4, and p. Accordingly, none of the dielectric compositions of the present invention should be considered as being limited only to an oxygen mole ratio of 3. This is because the most important aspect of the dielectric compositions of the present invention is not whether the mole ratio of oxygen is 3, but, instead, whether the mole ratio of each different metal is prescribed within a certain range. Accordingly, in the dielectric compositions of the present invention, it should be noted that the mole ratio of oxygen is given as 3 for the convenience of avoiding unnecessary complexity in the compositional formulas.
The dielectric compositions of the present invention are characterized in that sintering can be improved without any deterioration of the high frequency characteristics, by adding KpNbO3 to a specific BMN system material and optionally incorporating a predetermined amount of another specified metal in a specific ratio.
When no KpNbO3 is added, the specific BMN (BaOxe2x80x94MgOxe2x80x94Nb2O5) system material cannot be sintered. Further, when the amount, x, of KpNbO3 is greater than 0.1, the Q-value (the no-load quality coefficient) is reduced. Thus, the sintering and dielectric characteristics can be reconciled by prescribing the ratio of KpNbO3 to BMN (BaOxe2x80x94MgOxe2x80x94N b2O5) material as x.
When the coefficient p of KpNbO3 is smaller than 1, it is difficult to sinter the specific BMN system material. When the coefficient p of KpNbO3 is greater than 2, the Q-value is reduced.
A higher Q-value can be obtained by prescribing the quantity of Ba occupying Ba sites within the composite perovskite compound, to a predetermined range. Specifically, the coefficient xcex1 of Ba is preferably set to be within the range from 0.9 to 1.3. When xcex1 is greater than 1.3, it is difficult to sinter the BMN system material. When xcex1 is smaller than 0.9, the Q-value is reduced. The coefficient xcex1 preferably ranges from 1.0 to 1.2, and, more preferably, ranges from 1.0 to 1.05. The temperature coefficient (xcfx84f) of the resonance frequency can preferably also be set as well as the Q-value.
The coefficient xcex2 of Mg preferably ranges from 0.3 to 0.35. When xcex2 is greater than 0.35, it is difficult to sinter the BMN system material. When xcex2 is smaller than 0.3, the Q-value is reduced. The coefficient xcex2 more preferably ranges from 0.31 to 0.33. The temperature coefficient (xcfx84f) of the resonant frequency can preferably also be set as well as the Q-value.
A higher Q-value can be obtained by using a material (referred to herein as a BMNSb material) in which an Nb site in the dielectric composition of the present invention is partially replaced with Sb. The coefficient xcex4 of Sb is preferably set to be equal to or smaller than 0.125. When xcex4 is greater than 0.125, sintering is more difficult and the reproducibility of the desirable characteristics is also reduced. The coefficient xcex4 more preferably ranges from 0.05 to 0.075 since a high Q-value can then be obtained.
The temperature coefficient (xcfx84f) of the resonance frequency (which can approach zero) can be further improved by partially replacing the B-site of the perovskite crystal structure with Sn in the BMNSb system material. The quantity y of Sn preferably ranges from 0.15 to 0.3 since the temperature coefficient xcfx84f can be adjusted to within xc2x110. An excellent value almost near 0 ppm/K in the temperature coefficient xcfx84f is obtained by setting the quantity y of Sn to be within the range of 0.22 to 0.23 (and, more preferably, at 0.225).
The following Examples illustrate the invention but should not be considered as limiting the scope thereof.